As shown in FIG. 1, a power supply circuit 10 comprises a NMOSFET switch M1 as a power switch coupled between an input voltage VIN and an output VOUT. When charge pump 12 pulls high the gate voltage of the NMOSFET switch M1 over a threshold, the NMOSFET switch M1 turns on and thereby the input voltage VIN is coupled to the output VOUT. When the charge pump 12 stops working, and hence the gate voltage of the NMOSFET switch M1 drops down to be lower than the threshold, the NMOSFET switch M1 turns off, thereby disconnecting the input voltage VIN from the output VOUT. However, the charge pump 12 pulls high the gate voltage of the NMOSFET switch M1 from zero to the threshold requires a response time, which degrades the switching speed of the NMOSFET switch M1 and hence the operation speed of the power supply circuit 10.
Therefore, it is desired a quick turn on apparatus and method for a NMOSFET switch.